Among secondary batteries, a nonaqueous electrolyte secondary battery is a secondary battery that charges and discharges by transfer of lithium ion between positive and negative electrodes. Since a nonaqueous electrolyte secondary battery uses an organic solvent as an electrolytic solution, it may provide a larger voltage than a nickel-cadmium secondary battery and a nickel metal hydride secondary battery, both of which use an aqueous solution. A nonaqueous electrolyte secondary battery that is currently put into practical use uses a lithium-containing cobalt composite oxide or a lithium-containing nickel composite oxide as a positive electrode active material, a carbon-based material and lithium titanate or the like as a negative electrode active material, and uses a lithium salt such as LiPF6 and LiBF4 in the form of a solution in an organic solvent such as cyclic carbonates and linear carbonates as an electrolytic solution. The positive electrode active material has an average working potential of about from 3.4 to 3.8 V based on a metallic lithium potential, and the maximum potential during charging of from 4.1 to 4.3 V based on the metallic lithium potential. On the other hand, the carbon-based material and lithium titanate that are negative electrode active materials have average working potentials of about from 0.05 to 0.5 V and 1.55 V, respectively, vs a metallic lithium potential. By combining these positive and negative electrode active materials, the battery voltage becomes from 2.2 to 3.8 V, and the maximum charge voltage becomes from 2.7 to 4.3 V.
In order to further improve a capacity, use of LiMn1.5Ni0.5O4 that provides the maximum potential during charging of from 4.4 to 5.0 V for a positive electrode is considered. However, in a positive electrode comprising LiMn1.5Ni0.5O4, a carbonate-based solvent causes an oxidation reaction during charging, whereby causes deterioration of cycle performance and generation of gas. Furthermore, sultone and sulfone-based compounds have high viscosity, and the potential of oxidative decomposition is increased, but reactivity with a solvent in a negative electrode is increased. Thus, excellent cycle performance may not be obtained.